I have a Zoom H6. It's plugged in to my computer and when I make a recording for my podcast I have GarageBand running on my mac capturing all the audio. Can I have the SD card in the Zoom recording and saving the audio while I also have it recording and saving in GarageBand on my computer? I want to create an automatic backup on the card while I am recording it on my mac.

My first thought was the answer is likely no but I found a sentence in the product description on Zoom's website that gives me hope:

There’s also a handy Loop Back mode that allows you to blend stereo signal coming from the computer or iPad with signal coming into the H6 inputs, useful for live streaming via the Internet or for adding narration to a musical backing track.

However, I haven't looked through the owner's manual to discover how to use Loop Back mode.

Okay, I'm not exactly sure what that loop back mode is but if it can do the job please let me know.

You could probably try it and get an answer faster than waiting for us to interpret the manual description (not all of us have the same access to the hardware -- my only field recorder is the H2n).

However, based upon page 71 of the manual, it will not work for what you want -- it will not record ON the H6. What it will do is mix the H6 inputs (microphones) with computer audio, and send the mix /back/ to the computer. For example, the computer is running an MP3 player (say), with the H6 I/F as the output device, the H6 microphones are used by the "announcer/DJ", and the mix is then sent back to the computer where some OTHER software is configured for H6 I/F as input device, and using computer speakers/disk drive/internet stream as its output device.

This may be a shot in the wild, not having an H6, but my H2n has a line out which is connected to the XY mics, regardless whether it is recording or not. I often use this to make recordings on the H2n while simultaneously recording the same audio with the PC or R24.
Why? During recording sessions the H2n is continuously recording everything while the R24 records the takes.

This may be a shot in the wild, not having an H6, but my H2n has a line out which is connected to the XY mics, regardless whether it is recording or not. I often use this to make recordings on the H2n while simultaneously recording the same audio with the PC or R24.
Why? During recording sessions the H2n is continuously recording everything while the R24 records the takes.

True. But as discussed earlier in another thread I once compared a couple of recordings and found no differences.
I tested this by adding the signals in counterphase wich led to a practically zero result. As far as I can tell this indicates that the loss due to the conversions is neglectible.